1. Field of the Invention
The present invention relates to a dot line printer having a plurality of print hammers which are selectively fired to make impressions of dots. More particularly, the invention relates to a method and apparatus for controlling a print speed depending on a print density to prevent overheating of the manner assembly.
2. Description of the Prior Art
FIG. 1 schematically shows a dot line printer. Although not illustrated in FIG. 1, a plurality of print hammers are mounted on a hammer bank 3. The hammer bank 3 is driven by a shuttle motor 1 through a cam 2 and is reciprocally moved between two extremes to traverse a print paper. With a single rotation of the motor 1, the hammer bank 3 makes one way movement (one scan) and reverses at one of the two extremes. A rotary encoder 4 formed with a predetermined number of angularly spaced slits is mounted on a cam shaft. A photocoupler 5 consisting of a light emitting diode and a photodiode is disposed in association with the rotary encoder 4 for detecting an angular movement of the rotary encoder 4. A platen 6 is rotatably supported on a printer frame (not shown) for supporting the print paper thereon. A pair of pin tractors 9 are disposed in side marginal portions of the print paper, which upon engaging perforations formed on two sides of the print paper, move the print paper while cooperating with the platen 6. The print paper 8 is fed intermittently in a direction perpendicular to the direction in which the hammer bank 3 reciprocates. Both the platen 6 and the pin tractors 9 are driven by a paper feed motor 8.
FIG. 2 shows a print hammer and its drive unit. The print hammer 11 is in the form of an elongated leaf spring having an upper end to which a dot pin 12 is attached and a lower end fixedly secured through a front yoke 15 to a mounting plate 14 by a screw 16. A plunger 13 is also attached to the upper portion of the leaf spring. In association with each print hammer 11, there are provided a permanent magnet 17, a yoke 18, and an electromagnetic coil 19 wound around the yoke 18. The magnet 17 and the yoke 18 are elongated plate like members both extending in the direction parallel to the print line or perpendicular to the sheet of drawing so as to be commonly used by the plurality of print hammers 11. The upper portion of the hammer 11 is magnetically attracted to the pole of the yoke 18 and is released therefrom in response to print data causing to energize the coil 19. The dot pin 12 thus strikes the print paper through the ink ribbon 7, thereby making an impression of a dot on the paper. It is to be noted that the print hammer driver unit shown in FIG. 2 is shown by way of an example. Other types of driver units are also available.
The hammer assembly of the dot line printer generates heat with energization of the coil 19. To prevent the overheat of the hammer assembly, printing operation has been controlled with a print control circuit shown in FIG. 3. The circuit includes a counter 20 supplied with print data from a print data transmission circuit 10. The counter 20 counts a scheduled number of drivings at which the hammers 11 are to be driven during a forthcoming single scan of the hammer bank 3 wherein the hammer bank 3 moves from the first extreme to the second extreme or vice versa. The scheduled number of drivings is equal to a number of dots to be printed in one dot line.
A comparator 30 is connected to the output of the counter 20. The comparator 30 has a first input supplied with the output of the counter 20, i.e., the scheduled number of drivings, and a second input supplied with a reference number from a data latch circuit 40. The comparator 30 performs two stage comparisons wherein the comparator 30 first compares the scheduled number of drivings with the reference number, and thereafter the scheduled number of drivings with another reference number twice as large as the reference number latched in the data latch circuit 40. The comparator 30 outputs a print mode change instruction which indicates one of three alternatives, one being a status wherein the scheduled number of drivings is smaller than the reference number, second being a status wherein the scheduled number of drivings is larger than the reference number but smaller than twice the reference number, and third being a status wherein the scheduled number of drivings is larger than twice the reference number. A control unit 50 is connected to the output of the comparator 30 and controls a print hammer drive circuit 70 in response to the print data supplied from the print data transmission circuit 10 and the print mode change instruction from the comparator 30. The print hammer drive circuit 70 drives the print hammers in a divided mode when the print mode change instruction indicates that the scheduled number of drivings is greater than the reference number. When the print mode change instruction indicates the second alternative, printing of dots in one dot line is performed with two scans of the hammer bank 3. When the print mode change instruction indicates the third alternative, printing of dots in one dot line is performed with four scans of the hammer bank 3. In the divided mode, one dot line is not completely printed until the hammer bank 3 moves with forward and backward movements for several times. By printing in the divided mode, the number of drivings at which the hammers are driven in one scan of the hammer bank 3 can be reduced to less than the reference number set in the circuit 40.
The reference number is set to indicate an upper limit of an allowable number of dots that can be printed in one dot line. The circuit shown in FIG. 3 controls a dot line print density with one scan of the hammer bank 3 so as not to exceed a value obtained by dividing the reference number set in the circuit 40 with the maximum number of dots that can be printed in one dot line. More specifically, if M dots are printable at maximum in one dot line, the printer is allowed to perform normal printing when one line print density is less than m/M wherein m indicates the scheduled number of hammer drivings in one dot line. As shown in the timing chart of FIG. 4, the conventional printer sets m/M to 25%. When m/M exceeds 25% but below 50%, the control unit 50 controls the print hammer drive circuit 70 to perform printing in a two-divided mode wherein one dot line printing is accomplished with two scans of the hammer bank 3. When m/M exceeds 50%, the control unit 50 controls the print hammer drive circuit 70 to perform printing a four-divided mode wherein one dot line printing is accomplished with four scans of the hammer bank 3. Therefore, in the two- or four-divided modes, a ratio of the dot number printed during one scan of the hammer bank 3 to the maximum number M does not exceed 25%.
The conventional print control circuit shown in FIG. 3 is disadvantage in that the print mode frequently shifts to the two- or four-divided mode even if high density printing does not last for a long time and therefore the overheating problem of the print hammer assembly does not occur. The frequent shifts to the divided mode delays the overall printing speed. One possible solution to this problem is to increase the critical values for shifting to the divided modes. However, if this is done, the cooling efficiency of a hammer cooling mechanism and heat radiation of the print hammer driving circuit 70 must be improved. Also electric power consumption increases attendant to the increase of the critical values.